1. Field of the Invention
The present invention relates to a method of controlling the transmission of messages between a plurality of stations, said stations being interconnected via a communication channel comprising at least an upstream channel at least partially shared by said stations. The upstream channel is divided into successive frames of predetermined length, each frame consisting of message slots, each of which includes a data part for the transmission of at least a message generated by one of the stations, and M minislots, for sending, in case of unsuccessful transmission of messages, transmission requests in the form of available data slot reservation signals. This method may for instance constitute an appropriate multiple access communication protocol for allowing low level interactivity in satellite networks.
2. Description of the Related Art
In addition to traditional television programmes, it is often proposed to offer to TV users various interactive services and especially the opportunity to exchange information with service providers. Interactivity will be available thanks to a low bitrate return link from user premises to service provider facilities via a geostationary satellite transponder. It is however a challenge to develop a technique that offers a simple and robust channel access to a large number of user terminals with varying traffic volumes, burstiness and time constraints.
A contention-based protocol well adapted to such applications is described for instance in U.S. Pat. No. 4,641,304. As illustrated in FIG. 1 showing an earth station 1 and several frames I, . . . , I+R, I+R+1, of a communication channel 2 shared by a plurality of ground stations (not shown), the earth station or hub 1, which may be one of these stations, constitutes a master station that is able to broadcast via a downstream link a feedback that monitors the upstream link of the channel. In the illustrated case, the round trip delay corresponding to a double hop between the stations and the hub (via a geostationary satellite) is equal to 0,54 second.
As shown in FIG. 2, said protocol, called ARRA for Announced Retransmission Random Access, is based on an organization of the upstream link (or channel) in frames of t(f) seconds in length, composed of K message slots 21(1) to 21(K) and a particular slot called common minislot pool or CMP. Each message slot (or data slot) has a time duration of t(m) and is composed of a data part (or data message DM) and K minislots, and the CMP is also composed of K minislots but without any data part. The time durations of the minislots of the CMP and of each message slot are very small compared to t(m), and the minislots of each message slot occur at the beginning of the message slot.
When a message has to be transmitted from a first station to a second one, this message has to wait until the beginning of the next frame, here called frame I, occurs. Before frame I, all the stations will have received from the hub a feedback message or signal indicating which slot is free for new messages and which is already reserved for messages which had already successfully sent a transmission request. If there are indeed some free message slots, the transmitting station chooses randomly one of these available slots, transmits its message in the data part of this slot, and puts an associated transmission request in one of the minislots (randomly chosen) of the concerned message slot, in order to announce where it will retransmit in case of collision. If there are no available slot, this station puts a transmission request in one of the minislots of the CMP.
After the propagation delay (0,27 second in the above-indicated example), the hub receives all the messages and transmission requests that have been sent in the frame I. It then detects collisions in the data slots and calculates the set of slots that are not reserved (or available slot set ASS) in the frame I+R+1, i.e. in the frame that begins after all the stations have received from the hub the feedback messages (all the stations that have transmitted in frame I being assumed to receive their feedback before frame I+R+1). A slot in said frame I+R+1 is said to be reserved if and only if one request has announced a retransmission in this slot, and on the contrary is available if no request has announced such a retransmission. If more than one request announces a retransmission in the same slot, no successful transmission is possible and this slot is let free for new arrivals, while retransmission requests now occur in the CMP.
After having calculated the available slot set (ASS), i.e. the set of slots available for new messages, the hub broadcasts to all the stations both said ASS and the status of each message slot in the frame (empty, or available=E; used with success=S; collision=C):
(1) if a station has sent a message in a data slot and receives a feedback signal S (successful transmission), it exits the protocol; PA1 (2) if a station receives a feedback signal C (collision) or has sent a request in the CMP, it reads the ASS broadcasted by the hub:
(a) if the reference number of the minislot does not correspond to any of the slots of the ASS, it means that this number corresponds to a reserved slot and that the request for retransmission has been successful: the station sends its message in this reserved data slot, and will then exit the protocol; PA2 (b) if the reference number of the minislot corresponds to a slot of the ASS, it means that the request has failed, and this slot is available for new messages, while the concerned station has to use the CMP for retransmitting its request.
Some computations related to the implementation of this ARRA protocol show that the throughput cannot overpass 0,4 (i.e. 40% of the data slots are not used for transmission). At high loads, more and more requests are indeed sent in the CMP and it becomes impossible to transmit a successful request because of the high probability of collision, in the CMP, between a high number of retransmitted requests trying to use a relatively small amount of minislots.